Minimal code to write a c# windows form:

using System.Windows.Forms;

namespace ConsoleFormApp
{
    class MyWindow : Form
    {
        public MyWindow()
        {
            this.Text = "First Form";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Application.Run(new MyWindow());
        }
    }
}